1. Field of the Invention
This invention relates generally to voltage detecting circuits, and more particularly, to a voltage detecting circuit for detecting the presence of an input voltage which is outside the range of the supply voltages of the circuit.
2. Description of the Prior Art
In general, voltage detecting circuits are designed to provide a particular output in response to the presence of an input signal which is within the range of the supply voltages of the circuit. However, in some applications, it is desirable to detect the presence of input voltages which are outside the range of the supply voltages of the detecting circuit. For example, in a typical monolithic microprocessor having a electrically programmable read only memory (EPROM), the supply voltages of the microprocessor will be +5 volts and 0 volts. However, the on-chip EPROM can only be programmed using a programming voltage well outside of these supply voltages. In a typical N-channel device, the programming voltage will be of the order of +20 volts, while in a CMOS device the programming voltage would be of the order of -15 or +20 volts depending upon the conductivity type of the field effect transistors used to form the EPROM storage cells. If the microprocessor is also of the self-programming type, i.e. capable of automatically transfering the contents of an external memory into the on-chip EPROM, some means must be provided to force the device into the self-programming mode. One convenient way for performing this function is to provide a voltage detecting circuit which provides a distinctive output signal only when the programming voltage is applied to a particular input pin of the microprocessor. However, prior art voltage detecting circuits of this type have tended to be of the rather complicated voltage comparator form. In addition, the output signal which such prior art voltage detection circuits provide to indicate the presence of the programming voltage has typically been restricted to the range of the supply voltages.